Pulse oximeters and tissue oximeters are medical devices that measure absorption of near infrared light to determine blood oxygen saturation. Sometimes, pulse oximeters and tissue oximeters are used simultaneously on a patient's body. Typically, pulse oximeters use a disposable sensor placed on a peripheral site of the body where arterial capillary blood pulsation is high, such as on the finger. The sensor uses one or more light emitting diodes (LEDs) to emit light with two wavelengths and silicon photodetectors to measure light intensity transmitted through or reflected back from the site. To separate the two wavelengths of light, pulse oximeters usually use a single frequency to excite each LED in two different phases shifted by 180 degrees. By analyzing the AC of the reflected light intensities at two different wavelengths separately, pulse oximeters estimate arterial blood oxygen saturation (aSpO2). Because the arterial blood has uniform oxygen saturation across the whole body, a single sensor is usually employed on the patient to measure arterial blood oxygen saturation.
Tissue oximeters, including a cerebral oximeter, analyze the DC components of the reflected light at multiple wavelengths to determine oxygen saturation. Tissue oximeter sensors employs light from one or more LEDs or laser diodes and uses silicon photodiodes, avalanche photodiodes, or similar detectors to measure light absorption. In order to measure absorption of the light that travels deep inside the tissue or organs, tissue oximeters use a large separation between emitters and detectors. The measured light absorption related to the oxygen status of the tissue or organs is called the regional oxygen saturation (rSO2). Because different sites of the body have different values of regional oxygen saturation, multiple sensors on multiple body locations are usually employed to measure patient status. To measure light intensity with a high signal to noise ratio, the tissue oximeter employs a synchronous demodulation technique. To separate light signals from multiple wavelengths and from multiple sensors, tissue oximeters use time division or frequency division multiplexing or similar methods
Because both pulse and tissue oximeters use near infrared light, light generated by one type of oximeter can interfere with the other type of oximeter. This interference can happen regardless of where the sensors are placed on the patient's body. For example, light from one sensor can be detected by the light detectors of another sensor by: (1) propagating directly inside the tissue, (2) exiting from the tissue under the sensor and reflecting from objects surrounding the patient back to the tissue. Interference may be especially common during infant or pediatric monitoring where, for example, a forehead reflectance pulse oximeter sensor and cerebral oximeter sensor are used in close proximity.
Pulse oximeters and tissue oximeters, however, do not use continuous sinusoidal signals for modulation-demodulation. Rather, pulse and tissue oximeters use pulses with spectrums that may include multiple harmonics of a fundamental frequency. The harmonics of the fundamental frequency of one oximeter can fall in a pass band of a demodulator of another oximeter creating interference.
The severity of interference may depend upon differences between the modulation frequencies of the oximeters, a width of the pulses, and the pass band of a filter of the demodulator. The pulse oximeter may be configured to reject multiple harmonics of an AC component of ambient light and/or a main power line. The modulation frequency f1 of the pulse oximeter may be related to the frequency of the main power line (e.g., 50 Hz or 60 Hz). For example, the modulation frequency f1 may be equal to 1365 Hz so that differences between the modulation frequency f1 and the 22nd harmonic of ambient light having a frequency of 60 Hz (i.e., 1365−22*60 Hz=45 Hz) will not fall inside the pass band of the demodulator that usually is equal to the band frequency of plethysmogram (e.g., F=7.5 Hz).
However, light pulses from the pulse oximeter at a frequency (e.g., f1) of, for example, 1365 Hz may be received by and interfere with the tissue oximeter sensor. In particular, the light pulses may interfere with the tissue oximeter sensor light pulses depending on modulation frequency (e.g., f2) of the tissue oximeter pulses.
Because of low light return in the tissue oximeter sensor in comparison to the pulse oximeter sensor, the duration of the light pulses used for the tissue oximeter may be much more than for the pulse oximeter (e.g., f2 is less than f1). For example, the modulation frequency f2 of the tissue oximeter pulses may be 15 Hz with a pulse duration of 1 millisecond while the frequency f1 of the light pulses from the pulse oximeter may be 1365 Hz. The tissue oximeter may use a synchronous demodulator to multiply the signal from the photodiode by +1 or −1 depending on a phase of the light source excitation. Because of this, the synchronous filter may successively demodulate all harmonics of the modulation frequency f2. We can see that the 91st harmonic of the tissue oximeter will interfere with the pulse oximeter (e.g., 15 Hz*91=1365 Hz). Thus, a pulse oximeter signal at 1365 Hz will be demodulated by the tissue oximeter as a DC signal. In practice because the interfering frequency fi is subject of the slight variation from 15 Hz*91 due to low frequency fluctuations of the system clock of the pulse oximeter CPU, the DC surplus from the interfering light will show signs of low frequency variations. In some cases these low frequency variations can be significant and erroneously interpreted as a physiological effect.
Interference may occur with other values of f1 and f2, and separation of f1 and f2 does not assure absence of interference between two oximeters. Accordingly, a new system is needed that eliminates interference.